The present invention relates to a high speed automated machine for the manufacture of printing masters by electrophotographic means utilizing modulated laser light as the exposure means.
Significant advances in the art of printing plate technology have occurred in recent years. Printing masters such as have been employed in lithographic offset or direct printing processes are normally prepared by the imagewise exposure of a photosensitive coating which has been applied to a suitable support. Typical of such coatings are the so-called positive acting diazos, as for example disclosed in German Patent Specification No. 854,890, which undergo photodecomposition in the areas of the coating exposed to a source of actinic light, which exposed areas may subsequently be removed by treatment with a liquid developer solution in which only the photodecomposed areas are soluble. The negative acting coatings, on the other hand, undergo a photohardening or photopolymerization in those areas exposed to actinic light and only the unexposed areas of the coating are subsequently removed by appropriate developer. Representative of such negative acting materials are para quinone diazides such as disclosed in German Patent Specification No. 960,335, or condensation products of diazonium salts such as disclosed in U.S. Pat. Nos. 3,679,419; 3,867,147; and 3,849,392.
Offset plates have also been prepared by electrophotographic methods. Such plates are normally composed of a photoconductive material such as zinc oxide or cadmium sulfide dispersed in an ink-repelling binder material and coated on a suitable base material such as paper, metal or a film. These plates are imaged by the normal electrophotographic process involving forming an electrostatic charge on the surface of the plate, exposing the charged plate on an electrically conductive support to an image pattern of electromagnetic radiation, developing the resulting electrostatic image pattern by contact with an electroscopic liquid or solid developer, and fixing the developed image by drying or heating. The resultant imaged plate may be then used as a master for offset lithographic printing. An example of a machine for automatically performing such an electrophotographic process is disclosed in U.S. Pat. No. 4,006,984.
Because of the increased use in recent years of electronic methods for recording, storing and/or generating information such as by computers, cathode ray tubes, facsimile devices and the like, there have been some advances in the modification of the state of the printing plate art and the compatibilization of plate making processes with the newer technology for generating image information. For example, U.S. Pat. No. 3,549,733 discloses the use of a modulated high intensity 30 watt carbon dioxide gas laser to image a printing plate wherein polymeric material on the plate surface is decomposed to form ridgeless depressions, thus forming a relief plate. U.S. Pat. No. 3,506,779 discloses a laser beam typesetting apparatus for forming relief plates wherein a high intensity 100 watt carbon dioxide laser is utilized to remove plate material from the plate surface by vaporization. U.S. Pat. No. 3,664,737 teaches a printing plate recording system involving direct laser exposure of diazo sensitized printing plates which are subsequently developed by conventional development methods. An example of a process for manufacturing printing masters by photochemical means utilizing a relatively high powered 15 watt exposure laser is the LASERITE.RTM. system of the Eocom Corporation of Irvine, Calif., which process is described in the Mar. 10, 1975 publication, "The Seybold Report" by Seybold Publications.
In spite of the advanced made in the automation of platemaking technology, most of the processes and apparati presently available which utilize modulated laser light as the source of light exposure are relatively slow with regard to their platemaking capability, requiring anywhere in the range of about 2 to 40 minutes or more to process a single unexposed master into a finished plate ready for offset printing. Also, many of the known processes and machines rely on the use of relatively high powered output lasers, i.e. greater than 1 watt and often 15 watts or more, in order to accomplish the work of exposing, etching or deforming plate surfaces. Aside from the high energy requirements of such lasers, there are attendant problems in providing adequate cooling means which adds bulk and expense to the apparatus in which such lasers are embodied.
Also, in a system such as disclosed in U.S. Pat. No. 4,006,984, referred to above, electrophotographic plates are charged by mechanically passing a corona charging device such as known in the art over the surface of the plate, after which the plate is exposed by a full frame photographic exposure. Because the entire plate surface is exposed at once, the decay of electrostatic charge on the plate surface is of little moment. However, with a raster scan laser system wherein the laser scan line advances slowly over the plate surface, some of the electrostatic charge present on the end of the plate opposite the advancing scan line may decay prior to exposure, resulting in a noticeable image density differential in the copy after development.
Accordingly, it is an object of this invention to provide an electrophotographic imaging process and apparatus for producing printing masters utilizing modulated laser light as the exposure means.
Another object is to provide an apparatus for the electrophotographic production of finished printing masters at the rate of about one master per minute.
Another object is to provide an apparatus which is adapted to automatically convey, electrostatically charge, laser expose, electrostatically develop and finish electrophotographic printing masters for use in offset or lithographic printing processes.